1. Technical Field
The invention generally relates to power supplies and, more particularly, to a power supply that performs power factor correction with series interleaved phasing.
2. Related Art
Power supplies are utilized as a source of power in many electrical devices including most devices having electronic circuits. A power supply may utilize input power from a single phase or a multiple phase alternating current source to produce output power. The output power may be produced at one or more predetermined voltages with a determined range of output current. The output power may be alternating current (AC) or direct current (DC) of almost any magnitude depending on the load the power supply is serving.
Some power supplies and associated electrical device loads may be classified as non-linear power electronic loads. Such non-linear power electronic loads typically include rectifier/capacitor input stages that are characterized by an undesirably low power factor due to excessive load current harmonics. Load current harmonics cause an increase in the magnitude of RMS current supplied to such a non-linear power electronic load. Harmonic currents result in a reduction in power factor since harmonic currents do not provide useful power to the non-linear power electronic load.
Multiple kilowatt non-linear power electronic loads, such as a high power audio amplifier or a magnetic resonance imaging gradient amplifier, place significant current demands on a source of input power. A power feed from a source of input power may be supplied to a load from a circuit breaker with limited current carrying capacity. For example, a power feed that is a single phase power distribution system may be supplied from a circuit breaker that is rated for fifteen amps of sustained RMS current at near unity power factor. When a load with a low power factor is present, the RMS current requirement is higher, and the circuit breaker may open the power feed even though the load is not productively utilizing substantial power.
Power factor correction (PFC) may be used to decrease the magnitude of additional RMS current resulting from harmonics. Power factor correction may involve working to maintain the sinusoidal waveform of current drawn from an AC power source in phase with the sinusoidal waveform of voltage drawn from the AC power source. For non-linear power electronic loads, there are passive and active power factor correction approaches. Passive approaches include series inductor filters and resonant filters. Active approaches include boost derived converters and other switch mode based systems.
In general, boost derived converters utilize switching frequencies higher than the frequency of the source of input power (typically 50–60 Hz) to control the shape of the input current waveform. The higher switching frequencies may result in undesirably high levels of ripple frequencies (e.g. distortion). In addition to power factor correction, boost derived converters that are referred to as universal input boost converters have the capability to accept a range of input voltages such as 100 VAC nominal (Japan), 120 VAC nominal (United States) and 230 VAC nominal (Europe). Boost derived converters may also provide voltage regulation of the output voltage of the converter.
Power supplies that include a boost derived converter may include a bridge rectifier, a second stage, and a first stage that includes an inductor, a switch, a diode and a capacitor. AC power is rectified by the rectifier and used to magnetize the inductor. The switch is opened and closed with a high frequency time varying duty cycle to magnetize and demagnetize the inductor. The capacitor is charged with the energy discharged from the inductor during the demagnetization portion of the duty cycle. The voltage across the capacitor is a DC boost voltage that is provided to the second stage. The second stage converters the DC boost voltage to a DC output voltage of the power supply.
One type of boost converter known as a three-level boost converter may include an output having a capacitor voltage divider. The three-level boost converter includes an inductor and a pair of boost sub-circuits. The pair of boost sub-circuits are electrically connected in series and each include a switch, a capacitor and a diode. The series connection allows reduction in the voltage rating of the boost derived converter to half that of other boost derived converters since each boost sub-circuit operates to provide half of the boost voltage. At light loads, however, equal voltage may not be produced across the capacitors. The voltage imbalance may result in unbalanced operation of the three level boost converter resulting in stress on the boost sub-circuits.
Some PFC boost derived converters operate in a discontinuous conduction mode (DCM) with switch mode operation. To minimize ripple current associated with such switch mode operation, some boost derived converters, such as the three-level boost converter, operate with interleave. Interleave operation involves multiple switches in the boost derived converter that are operated sequentially during a switching period to increase ripple frequency while reducing ripple magnitude. The reduction in ripple magnitude further decreases undesirable line currents and therefore improves power factor. Increased ripple frequency results in cancellation of ripple current at the switching frequency, at sidebands of the switching frequency, at odd harmonics of the switching frequency and at sidebands of the odd harmonics. The reduced magnitude of ripple current, however, still creates undesirable load currents. In addition, with the previously discussed unbalanced conditions of the three-level boost converter, reduction in ripple current may be adversely affected due to incomplete ripple cancellation.
Therefore a need exists for a power factor correcting power supply with greater power efficiency and lower ripple currents that does not suffer from internal voltage imbalances.